1. Field of the Invention
This invention relates to a method of adjusting radiation image read-out conditions used in a radiation image recording and reproducing system. This invention particularly relates to a method of adjusting the final read-out conditions on the basis of image information obtained by preliminary read-out in a radiation image recording and reproducing system wherein the preliminary read-out and the final read-out are carried out.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
One embodiment of the aforesaid radiation image recording and reproducing system is disclosed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the embodiment, before final read-out is conducted by scanning the stimulable phosphor sheet carrying a radiation image of an object stored therein by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, several steps are carried out. The emitted light is detected by a photoelectric read-out means and converted into an electric image signal. These steps produce a preliminary read-out for approximately detecting the image information stored in the stimulable phosphor sheet by use of stimulating rays of a level lower than the level of the stimulating rays used in the final read-out. Read-out conditions for the final read-out are adjusted on the basis of the information obtained by the preliminary read-out, and the final read-out is conducted by use of the read-out conditions. The electric image signal obtained by the final read-out is sent to an image processing means and is processed in accordance with the image recording portion of the object and/or the image recording method to obtain a visible image suitable for viewing, particularly for diagnostic purposes. The processed image signal is used to reproduce the visible image on a photographic film or the like.
As described above, the level of the stimulating rays used in the preliminary read-out is lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out. In order to make the level of the stimulating rays used in the preliminary read-out lower than the level of the stimulating rays in the final read-out, the output of the stimulating ray source such as a laser beam source may be decreased in the preliminary read-out, or the stimulating rays emitted by the stimulating ray source may be attenuated by an ND filter, an AOM, or the like positioned on the optical path. Alternatively, a stimulating ray source for the preliminary read-out may be positioned independently of the stimulating ray source for the final read-out, and the output of the former may be made lower than the output of the latter. Or, the beam diameter of the stimulating rays may be increased, the scanning speed of the stimulating rays may be increased, or the moving speed of the stimulable phosphor sheet may be increased in the preliminary read-out.
When the image information stored in the stimulable phosphor sheet is approximately determined prior to the final read-out and the final read-out is conducted by use of the read-out conditions adjusted on the basis of the image information, it becomes possible to eliminate adverse effects of a fluctuation in the level of the radiation energy stored in the stimulable phosphor sheet, which is caused by a change in the object or the image recording portion thereof, by a fluctuation in the radiation dose, or the like, and to conduct the final read-out by use of desirable read-out conditions.
Various methods may be used as the preliminary read-out for approximately determining the image information prior to the final read-out and adjusting the read-out conditions. One method is to carry out preliminary read-out by scanning the stimulable phosphor sheet by stimulating rays of a low level and detecting the light emitted by the sheet during the scanning, to determine a histogram of the amount of emitted light, and to adjust the read-out conditions on the basis of the histogram.
In U.S. patent application No. 695,332, the applicant proposed a novel method of adjusting the read-out conditions on the basis of the histogram. The method comprises the steps of: calculating the maximum light emission amount Smax and the minimum light emission amount Smin of a desired image information range in the histogram, and adjusting the read-out conditions so that the maximum light emission amount Smax and the minimum light emission amount Smin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin of a desired input signal range in the image processing means which are determined by the maximum density Dmax and the minimum density Dmin of a correct density range in the reproduced visible image.
However, the histogram of the light emission amount obtained by the preliminary read-out (hereinafter referred to as the preliminary read-out histogram) often includes noise generated by various causes. For example, when a region unexposed to X-rays is present on the stimulable phosphor sheet, it has been assumed that the unexposed region does not emit light when exposed to stimulating rays. Actually, however the unexposed region stores energy of scattered X-rays and emits light in some amount when exposed to stimulating rays. In this case, the preliminary read-out histogram obtained includes noise caused by the scattered X-rays.
As, therefore, the preliminary read-out histogram often includes noise, in the method of directly adjusting the read-out conditions on the basis of the preliminary read-out histogram without pre-processing the histogram, it is not always possible to determine Smax and Smin accurately and consistently and to adjust the read-out conditions accurately and consistently.